I. Field of the Invention
This invention relates generally to a method and apparatus for assessing patient well-being, and more particularly to a method and apparatus for indirectly determining a patient's peak oxygen uptake (VO2.sub.MAX) by measuring his/her heart rate variability (HRV).
II. Discussion of the Prior Art
It is known in the art that HRV, i.e., the beat-to-beat variance in sinus cycle length over a period of time, is a predictor of mortability and morbidity. Patients exhibiting low HRV show a significantly increased risk of sudden cardiac death. See "Heart Rate Variability" by Zsolt Ori, et al., Cardiology Clinics, Vol. 10, No. 3, August 1992, pp. 499-537 and "Depressed Heart Rate Variability As An Independent Predictor of Death in Chronic Congestive Heart Failure Secondary to Ischemic or Idiopathic Dilated Cardiomyopathy" by Piotr Ponikowski, et al., The American Journal of Cardiology, Vol. 79, Jun. 15, 1997, pp. 1645-1650. See also "Correlations Among Time and Frequency Domain Measures of Heart Period Variability Two Weeks After Acute Myocardial Infarction" by J. Thomas Bigger et al., The American Journal of Cardiology, Vol. 69, Apr. 1, 1992. In addition, the Spinelli et al. U.S. Pat. No. 5,466,245 provides a very detailed method for automatically determining AV delay based on evaluating a frequency domain measure of heart rate variability (HRV).
In the Heemels et al. U.S. Pat. No. 5,603,331, assigned to applicant's assignee, a method and apparatus is described for efficiently processing, logging and disseminating essential features relating to HRV accumulated from a continuous, long-term monitoring of cardiac activity. As pointed out in that patent, the method sufficiently conserves data memory, program memory and power consumption that it may be incorporated within an implantable pacemaker or defibrillator to log a 24-hour period of cardiac activity accumulated for subsequent telemetry to an external monitor. The patent further describes a method of processing and displaying HRV data in a manner that is readily understandable by clinicians which provides an improved graphical contrast between normal and abnormal HRV patterns.
Further studies which we have recently conducted on several patients has revealed a high correlation between HRV and a patient's exercise capacity as measured by VO2.sub.MAX. Based upon this realization, a method has been established for indirectly assessing a patient's maximum oxygen uptake by computing the standard deviation of five minute mean RR intervals, i.e., the SDANN Index. In an alternative approach, a two-dimensional histogram array is plotted with RR intervals along one axis and the absolute value of the time difference between successive RR intervals plotted along the second axis. By measuring the area on the plot occupied by the two-dimensional histogram, VO2.sub.MAX can be estimated.